During production of kraft pulp, black liquor is formed and removed from the produced pulp. The removed black liquor comprises soap which needs to be separated from the black liquor since the soap comprises valuable raw materials. Another reason to separate the soap from the black liquor is that the soap may cause problems during subsequent treatment steps of the black liquor.
The separated soap comprises crude tall oil (CTO), water, lignin, inorganic compounds and fibers. The CTO is a mixture of fatty and resin acids and unsaponifiables. The fatty and resin acids of CTO are in the form of sodium salts in the soap. The amount of each component in the soap depends on the raw material used during the pulping process and on the recovery process in which the soap is separated from the black liquor, i.e. the soap skimming process.
Crude tall oil is a valuable raw material and it is important to recover as much of the crude tall oil from the soap as possible. Crude tall oil can be used as a raw material for various chemicals and other products, e.g. biodiesel or detergents.
It is possible to separate the CTO from the soap by addition of an acid to the soap at certain temperature. After mixing of the soap and the added acid an acidulated soap is formed and the acidulated soap separates then into three major phases due to density differences of the phases; a CTO phase, a lignin phase and a spent acid phase. The lignin and spent acid phase are rejects in the CTO production and they need to be separated well from the CTO phase during the recovery of the CTO.
The amount of acid needed in order to separate the optimal amount of CTO from the soap depends on the quality of the soap, e.g. the CTO content, the water content, the fiber amount, the lignin content and/or the black liquor content. Today it is common to measure the density of the soap, and the pH and density of the spent acid as a measure of the amount of acid and water that needs to be added in order to separate the optimal amount of the CTO from the soap. These measurements are done online and the needed amount of acid and water is thereafter adjusted, i.e. feed back control.
The methods used today are not very sensitive and fast enough to adjust the needed amount of acid and water when the quality of the soap changes. There is thus a need for an improved method for the control of the recovery of CTO from a soap.